Signal Source

A Signal Source is the block that injects a continuous stream of raw samples of GNSS signal to the processing flow graph. This is an abstraction that wraps all kind of sources, from samples stored in files (in a variety of formats) to multiple sample streams delivered in real-time by radio frequency front-ends.

The input of a software receiver are the raw bits that come out from the front-end’s analog-to-digital converter (ADC), as sketched in the figure below. Those bits can be read from a file stored in the hard disk or directly in real-time from a hardware device through USB or Ethernet buses.

Front-end
Simplified block diagram of a generic radio frequency front-end, consisting of an antenna, an amplification stage, downshifting from RF to and intermediate frequency (or baseband), filtering, sampling, and an interface to a host computer for real-time processing mode, or to an storage device for post-processing.

The Signal Source block is in charge of implementing the hardware driver, that is, the portion of the code that communicates with the RF front-end and receives the samples coming from the ADC. This communication is usually performed through USB or Ethernet buses. Since real-time processing requires a highly optimized implementation of the whole receiver, this module also allows to read samples from a file stored in a hard disk, and thus processing without time constraints. Relevant parameters of those samples are the intermediate frequency (or baseband I&Q components), the sampling rate and number of bits per sample, that must be specified by the user in the configuration file, as shown below.

This block also performs bit-depth adaptation, since most of the existing RF front-ends provide samples quantized with 2 or 3 bits, while operations inside the processor are performed on 32- or 64-bit words, depending on its architecture. Although there are implementations of the most intensive computational processes (mainly correlation) that take advantage of specific data types and architectures for the sake of efficiency, the approach is processor-specific and hardly portable. We suggest to keep signal samples in standard data types and letting the compiler select the best library version (implemented using SIMD or any other processor-specific technology) of the required routines for a given processor.

For more details about sample formats, please check out our tutorial on data types in GNSS-SDR.

The more kinds of signal souces GNSS-SDR is able to work with, the better is its Interoperability.

Reading data from a file

The user can configure the receiver for reading from a file, setting in the configuration file the data file location, sample format, and the sampling and intermediate frequencies at which the signal was originally captured.

Real signals sampled at an intermediate frequency can be downshifted to baseband (and thus expressed as complex samples) by the Freq_Xlating_Fir_Filter implementation of the Input Filter present at the Signal Conditioner block with its IF parameter.

Implementation: File_Signal_Source

This Signal Source implementation reads raw signal samples stored in a file, as long as they are stored in one of the following formats: byte, ibyte, short, ishort, float or gr_complex. Their definition is as follows:

Type name in GNSS-SDR conf files Definition Sample stream
byte Signed integer, 8-bit two’s complement number ranging from -128 to 127. C++ type name: int8_t
short Signed integer, 16-bit two’s complement number ranging from -32768 to 32767. C++ type name: int16_t
float Defines numbers with fractional parts, can represent values ranging from approx. to with a precision of 7 digits (32 bits). C++ type name: float
ibyte Interleaved (I&Q) stream of samples of type byte. C++ type name: int8_t
ishort Interleaved (I&Q) samples of type short. C++ type name: int16_t
cbyte Complex samples, with real and imaginary parts of type byte. C++ type name: lv_8sc_t
cshort Complex samples, with real and imaginary parts of type short. C++ type name: lv_16sc_t
gr_complex Complex samples, with real and imaginary parts of type float. C++ type name: std::complex<float>

Data type definition in GNSS-SDR.

This implementation accepts the following parameters:

Parameter Description Required
implementation File_Signal_Source Mandatory
filename Path to the file containing the raw digitized signal samples Mandatory
sampling_frequency Sample rate, in samples per second. Mandatory
samples Number of samples to be read. If set to the whole file but the last two milliseconds are processed. It defaults to . Optional
item_type [byte, ibyte, short, ishort, float, gr_complex]: Sample data type. It defaults to gr_complex. Optional
repeat [true, false]: If set to true, processing of samples restarts the file when the end is reached. It defaults to false. Optional
enable_throttle_control [true, false]: If set to true, it places a throttle controlling the data flow. It is generally not required, and it defaults to false. Optional

Signal Source implementation: File_Signal_Source

This implementation assumes that the center frequency is the nominal corresponding to the GNSS frequency band. Any known deviation from that value can be compensated by using the IF parameter of the Freq_Xlating_Fir_Filter implementation of the Input Filter present at the Signal Conditioner block, or later on in the flow graph at the Acquisition and Tracking blocks with their if parameter.

It follows an example of a Signal Source block configured with the File_Signal_Source implementation:

;######### SIGNAL_SOURCE CONFIG ############
SignalSource.implementation=File_Signal_Source
SignalSource.filename=/home/user/gnss-sdr/data/my_capture.dat
SignalSource.sampling_frequency=4000000

Tip: The name of the file to be read (that is, SignalSource.filename) that appears on the configuration file can be overridden at the command line when invoking gnss-sdr with the flag --signal_source. Example:

$ gnss-sdr --config_file=/path/to/my_receiver.conf \
  --signal_source=/path/to/my_capture2.dat

This will read the configuration file my_receiver.conf, but it will read samples from the file my_capture2.dat instead of the one specified in SignalSource.filename.

Implementation: Two_Bit_Packed_File_Signal_Source

Sometimes, samples are stored in files in a format that is not in the list of “native” types supported by the File_Signal_Source implementation (i.e, it is not among byte, ibyte, short, ishort, float or gr_complex). This is the case of 2-bit real samples delivered at a given intermediate frequency, which is a common format for GNSS RF front-ends.

The Two_Bit_Packed_File_Signal_Source implementation allows reading two-bit length samples from a file. The data is assumed to be packed as bytes item_type=byte or shorts item_type=short so that there are 4 two bit samples in each byte. The two bit values are assumed to have the following interpretation:

b1 b0 Value
0 0 +1
0 1 +3
1 0 -3
1 1 -1

Within a byte the samples may be packed in big endian big_endian_bytes=true (if the most significant byte value is stored at the memory location with the lowest address, the next byte value in significance is stored at the following memory location, and so on) or little endian big_endian_bytes=false (if the least significant byte value is at the lowest address, and the other bytes follow in increasing order of significance). If the order is big endian then the most significant two bits will form the first sample output, otherwise the least significant two bits will be used.

Additionally the samples may be either real sample_type=real, or complex. If the sample type is complex, then the samples are either stored in the order: real, imag, real, imag, … sample_type=iq or in the order: imag, real, imag, real, … sample_type=qi.

Finally, if the data is stored as shorts item_type=short, then it may be stored in either big endian big_endian_items=true or little endian big_endian_items=false. If the shorts are big endian, then the second byte in each short is output first.

The output data type is either float or gr_complex depending on whether or not sample_type is real.

This implementation accepts the following parameters:

Parameter Description Required
implementation Two_Bit_Packed_File_Signal_Source Mandatory
filename Path to the file containing the raw digitized signal samples Mandatory
sampling_frequency Sample rate, in samples per second. Mandatory
samples Number of samples to be read. If set to the whole file but the last two milliseconds are processed. It defaults to . Optional
item_type [byte, short]: Sample data type. It defaults to byte. Optional
repeat [true, false]: If set to true, processing of samples restarts the file when the end is reached. It defaults to false. Optional
sample_type [real, qi, iq]: Set real or complex sample types (see above). It defaults to real. Optional
big_endian_bytes [true, false]: If set to true, the most significant byte value is expected to be stored at the memory location with the lowest address. If set to false, the least significant byte value is expected at the lowest address. It defaults to false. Optional
seconds_to_skip Seconds to skip in the file header. It defaults to s. Optional
big_endian_items [true, false]: If set to true, and the data is stored as shorts, it is interpreted as big endian. If set to false, data is interpreted to be stored in little endian. It defaults to true. Optional
enable_throttle_control [true, false]: If set to true, it places a throttle controlling the data flow. It is generally not required, and it defaults to false. Optional

Signal Source implementation: Two_Bit_Packed_File_Signal_Source

Example:

;######### SIGNAL_SOURCE CONFIG ############
SignalSource.implementation=Two_Bit_Packed_File_Signal_Source
SignalSource.filename=/data/my_capture.datz
SignalSource.item_type=short
SignalSource.sampling_frequency=60000000
SignalSource.samples=6000000000  ; Notice that 0 indicates the entire file.
SignalSource.repeat=false
SignalSource.dump=false
SignalSource.dump_filename=./signal_source.dat
SignalSource.enable_throttle_control=false
SignalSource.sample_type=iq
SignalSource.big_endian_items=true
SignalSource.big_endian_bytes=false

Implementation: Nsr_File_Signal_Source

Sometimes, samples are stored in files in a format that is not in the list of “native” types supported by the File_Signal_Source implementation (i.e, it is not among byte, ibyte, short, ishort, float or gr_complex). This is the case of 2-bit real samples delivered at a given intermediate frequency, which is a common format found in RF front-ends:

where are 2-bit real samples.

This Signal Source implementation is able to read such format and deliver at its output a sample stream composed of samples of type byte (8-bit signed integer). This implementation delivers a stream of samples of type gr_complex.

This implementation accepts the following parameters:

Parameter Description Required
implementation Nsr_Signal_Source Mandatory
filename Path to the file containing the raw digitized signal samples Mandatory
sampling_frequency Sample rate, in samples per second. Mandatory
samples Number of samples to be read. If set to the whole file but the last two milliseconds are processed. It defaults to . Optional
item_type [byte]: Sample data type. Only byte is allowed in this implementation. Optional
repeat [true, false]: If set to true, processing of samples restarts the file when the end is reached. It defaults to false. Optional
enable_throttle_control [true, false]: If set to true, it places a throttle controlling the data flow. It is generally not required, and it defaults to false. Optional

Signal Source implementation: Nsr_Signal_Source

It follows an example of a Signal Source block configured with the Nsr_Signal_Source implementation:

;######### SIGNAL_SOURCE CONFIG ############
SignalSource.implementation=Nsr_File_Signal_Source
SignalSource.filename=/datalogger/signals/ifen/E1L1_FE0_Band0.stream
SignalSource.item_type=byte
SignalSource.sampling_frequency=20480000
SignalSource.samples=0

Tip: The name of the file to be read (that is, SignalSource.filename) that appears on the configuration file can be overridden at the command line when invoking gnss-sdr with the flag –nsr_signal_source. Example:

$ gnss-sdr --config_file=/path/to/my_receiver.conf \
  --nsr_signal_source=/path/to/my_capture2.dat

This will read the configuration file my_receiver.conf, but it will read samples from the file my_capture2.dat instead of the one specified in SignalSource.filename.

Radio Frequency front-ends

Implementation: UHD_Signal_Source

Ettus Research The USRP Hardware Driver (UHD) software API supports application development on all Ettus Research USRP Software Defined Radio products. Using a common software interface is critical as it increases code portability, allowing applications to transition seamlessly to other USRP SDR platforms when development requirements expand or new platforms are available. Hence, it enables a significant reduction in development effort by allowing you to preserve and reuse your legacy code so you can focus on new algorithms.

This implementation accepts the following parameters:

Parameter Description Required
implementation UHD_Signal_Source Mandatory
device_address IP address of the USRP device. When left empty, the device discovery routines will search all the available transports on the system (Ethernet, USB, …) Mandatory
subdevice [A:0, B:0]: UHD subdevice specification. Mandatory
sampling_frequency Set the sampling frequency, in samples per second. Mandatory
RF_channels Number of RF channels present in the front-end device. It defaults to 1. Optional
clock_source [internal, external, MIMO]: Set the clock source for the USRP device. It defaults to internal. Optional
item_type [cbyte, cshort, gr_complex]: data type for each sample. The type cbyte (i.e., complex signed 8-bit integers) is not available in USRP devices with their default configurations. This parameter defaults to cshort. Optional

If RF_channels is set to 1, then:

Parameter Description Required
freq Set the RF front-end center frequency, in Hz. Mandatory
IF_bandwidth_hz Set the IF passband filter bandwidth of the front-end, in Hz. It defaults to sampling_frequency / 2. Optional
gain Set the RF front-end gain, in dB, distributed across all gain elements. It defaults to dB. Optional
samples Number of samples to be processed. It defaults to , which means infinite samples. Optional
dump [true, false]: If set to true, it enables the dump of the signal source delivered data into a file. It defaults to false. Optional
dump_filename If dump is set to true, name of the file in which internal data will be stored. It defaults to ./data/signal_source.dat Optional

Signal Source implementation: UHD_Signal_Source single-band parameters.

Example:

;######### SIGNAL_SOURCE CONFIG ############
SignalSource.implementation=UHD_Signal_Source
SignalSource.device_address=192.168.40.2  ; <- PUT YOUR USRP IP ADDRESS HERE
SignalSource.item_type=cshort
SignalSource.sampling_frequency=4000000
SignalSource.freq=1575420000
SignalSource.gain=40
SignalSource.subdevice=A:0
SignalSource.repeat=false
SignalSource.dump=false
SignalSource.dump_filename=../data/signal_source.dat
SignalSource.enable_throttle_control=false

If RF_channels is set to more than one, then the number of the radio-frequency channel (starting with ) is appended to the name of parameters samples, dump, dump_filename, freq, gain and IF_bandwidth_hz to indicate to which RF chain they apply.

For instance, if RF_channels is set to 2, then:

Parameter Description Required
freq0 RF front-end center frequency for RF channel 0, in Hz. Mandatory
IF_bandwidth_hz0 Set the IF passband filter bandwidth of RF channel 0, in Hz. It defaults to sampling_frequency / 2. Optional
gain0 Set the RF front-end gain for RF channel 0, in dB, distributed across all gain elements. It defaults to dB. Optional
samples0 Number of samples to be processed for RF channel 0. It defaults to , which means infinite samples Optional
dump0 [true, false]: If set to true, it enables the dump of the signal source delivered data into a file. It defaults to false. Optional
dump_filename0 If dump0 is set to true, name of the file in which data will be stored. It defaults to ./data/signal_source0.dat Optional
freq1 RF front-end center frequency for RF channel 1, in Hz. Mandatory
IF_bandwidth_hz1 Set the IF passband filter bandwidth of RF channel 1, in Hz. It defaults to sampling_frequency / 2. Optional
gain1 Set the RF front-end gain for RF channel 1, in dB, distributed across all gain elements. It defaults to dB. Optional
samples1 Number of samples to be processed for RF channel 1. It defaults to , which means infinite samples Optional
dump1 [true, false]: If set to true, it enables the dump of the signal source delivered data into a file. It defaults to false. Optional
dump_filename1 If dump1 is set to true, name of the file in which data will be stored. It defaults to ./data/signal_source1.dat Optional

Signal Source implementation: UHD_Signal_Source multiple-band parameters.

Tip: If the samples parameter is not specified, or set to , the USRP will deliver samples in a continuous way and with no specified end time, and so the software receiver will process endlessly. When configured for an infinite number of samples, please always terminate the software receiver execution by pressing key ‘q’ and then key ‘ENTER’. This will make the program to exit gracefully, doing some clean-up work and preparing output products such as RINEX files to be properly read by other software tools. This is not guaranteed if the program is interrupted for instance by pressing keys ‘CTRL’ and ‘c’ at the same time.

Implementation: Osmosdr_Signal_Source

OsmoSDR OsmoSDR is a 100 % Free Software based small form-factor inexpensive SDR (Software Defined Radio) project. It consists of USB-attached hardware, the associated firmware as well as software tools for GNU Radio integration. It provides a driver for several front-ends, such as RTL-based dongles, HackRF, bladeRF, etc.

If you installed GNSS-SDR from a software package, this implementation is already available. But if you built GNSS-SDR from the source code, you will need the required software dependencies (the gr-osmosdr component of GNU Radio) and configure the building with the following flag:

$ cmake -DENABLE_OSMOSDR=ON ../

For more information, check out the tutorial about GNSS-SDR options at building time.

This implementation accepts the following parameters:

Parameter Description Required
implementation Osmosdr_Signal_Source Mandatory
freq RF front-end center frequency, in Hz. Mandatory
gain RF front-end gain for RF channel 0, in dB. The value is spread across the receiving chain. It defaults to dB. Optional
rf_gain RF front-end gain for the RF amplifier, in dB. It defaults to dB. Optional
if_gain RF front-end gain for the IF amplifier, in dB. It defaults to dB. Optional
sampling_frequency Sampling frequency, in samples per second. It defaults to 2 Ms/s. Optional
AGC_enabled [true, false]: If set to true, enables Automatic Gain Control. It defaults to false. Optional
samples Number of samples to be processed. It defaults to , which means infinite samples. Optional
item_type [gr_complex]: Set the output data type. Only gr_complex is allowed in this version, so it is set by default. Optional
osmosdr_args Pass arguments to the OsmoSDR driver. Optional
dump [true, false]: If set to true, it enables the dump of the signal source into a file. It defaults to false. Optional
dump_filename If dump1 is set to true, name of the file in which data will be stored. It defaults to ./data/signal_source.dat Optional

Please note that not all the OsmoSDR-compatible devices can work as radio frequency front-ends for proper GNSS signal reception, please check the specifications. For suitable RF front-ends, you can use:

;######### SIGNAL_SOURCE CONFIG ############
SignalSource.implementation=Osmosdr_Signal_Source
SignalSource.item_type=gr_complex
SignalSource.sampling_frequency=2000000
SignalSource.freq=1575420000
SignalSource.gain=40
SignalSource.rf_gain=40
SignalSource.if_gain=30
SignalSource.enable_throttle_control=false
SignalSource.osmosdr_args=rtl_tcp,offset_tune=1

Implementation: RtlTcp_Signal_Source

In case of using a Zarlink’s RTL2832 based DVB-T receiver, you can even use the rtl_tcp I/Q server in order to use the USB dongle remotely. rtl_tcp is an I/Q spectrum server for RTL2832 based DVB-T receivers.

If you installed GNSS-SDR from a software package, this implementation is already available. But if you built GNSS-SDR from the source code, you will need the required software dependencies (the gr-osmosdr component of GNU Radio) and configure the building with the following flag:

$ cmake -DENABLE_OSMOSDR=ON ../

For more information, check out the tutorial about GNSS-SDR options at building time.

In a terminal, type:

$ rtl_tcp -a 127.0.0.1 -f 1575420000 -g 0 -s 2000000

and use the following configuration:

;######### SIGNAL_SOURCE CONFIG ############
SignalSource.implementation=RtlTcp_Signal_Source
SignalSource.item_type=gr_complex
SignalSource.sampling_frequency=2000000
SignalSource.freq=1575420000
SignalSource.gain=40
SignalSource.rf_gain=40
SignalSource.if_gain=30
SignalSource.AGC_enabled=false
SignalSource.samples=0
SignalSource.repeat=false
SignalSource.dump=false
SignalSource.dump_filename=../data/signal_source.dat
SignalSource.enable_throttle_control=false
SignalSource.address=127.0.0.1 ; Put your IP here
SignalSource.port=1234         ; Put your port here
SignalSource.swap_iq=false

Multiple radio frequency chains

A single Signal Source can be equipped with more than one radio-frequency chain. Examples of such configuration could be a USRP with two subdevices, or dual or triple band RF front ends, such as NSL Stereo or Flexiband.

This case implies not only the configuration of the Signal Source, but also there is a need to set up different Signal Conditioners for each band, and configure the Channel implementations for the different signals present on each band.

Multichannel
Simplified block diagram of a dual-band receiver of GPS L1 C/A and GPS L2C (M) signals.

The number of radio-frequency chains is denoted by parameter RF_channels, which defaults to one if it is not present in the configuration file.

SignalSource.RF_channels=2

Then:

SignalSource.RF_channels=2
SignalSource.implementation=UHD_Signal_Source
...
SignalSource.subdevice=A:0 B:0
...
SignalSource.freq0=1575420000
SignalSource.freq1=1227600000
...

SignalConditioner0.implementation=...
DataTypeAdapter0.implementation=...
InputFilter0.implementation=...
Resampler0.implementation=...

SignalConditioner1.implementation=...
DataTypeAdapter1.implementation=...
InputFilter1.implementation=...
Resampler1.implementation=...

...
Channels_1C.count=8
Channels_2S.count=8

; # Channel connection
Channel0.RF_channel_ID=1
Channel1.RF_channel_ID=1
Channel2.RF_channel_ID=1
Channel3.RF_channel_ID=1
Channel4.RF_channel_ID=1
Channel5.RF_channel_ID=1
Channel6.RF_channel_ID=1
Channel7.RF_channel_ID=1
Channel8.RF_channel_ID=0
Channel9.RF_channel_ID=0
Channel10.RF_channel_ID=0
Channel11.RF_channel_ID=0
Channel12.RF_channel_ID=0
Channel13.RF_channel_ID=0
Channel14.RF_channel_ID=0
Channel15.RF_channel_ID=0

; Channel signal
Channel0.signal=1C
Channel1.signal=1C
Channel2.signal=1C
Channel3.signal=1C
Channel4.signal=1C
Channel5.signal=1C
Channel6.signal=1C
Channel7.signal=1C
Channel8.signal=2S
Channel9.signal=2S
Channel10.signal=2S
Channel11.signal=2S
Channel12.signal=2S
Channel13.signal=2S
Channel14.signal=2S
Channel15.signal=2S

...


Acquisition_1C.implementation=...
; or Acquisition_1C0, ..., Acquisition_1C7
Acquisition_2S.implementation=...
; or Acquisition_2S8, ..., Acquisition_2S15


Tracking_1C.implementation=...
; or Tracking_1C0, ..., Tracking_1C7
Tracking_2S.implementation=...
; or Tracking_2S8, ..., Tracking_2S15


TelemetryDecoder_1C.implementation=...
; or TelemetryDecoder_1C0, ..., TelemetryDecoder_1C7
TelemetryDecoder_2S.implementation=...
; or TelemetryDecoder_2S8, ..., TelemetryDecoder_2S15

...

Example: Configuring the USRP X300 with two front-ends for receiving signals in L1 and L2 bands

;######### SIGNAL_SOURCE CONFIG ############
SignalSource.implementation=UHD_Signal_Source
SignalSource.device_address=192.168.40.2  ; Put your USRP IP address here
SignalSource.item_type=gr_complex
SignalSource.RF_channels=2
SignalSource.sampling_frequency=4000000
SignalSource.subdevice=A:0 B:0

;######### RF Channels specific settings ######
SignalSource.freq0=1575420000
SignalSource.gain0=50
SignalSource.samples0=0
SignalSource.dump0=false
SignalSource.dump_filename0=../data/signal_source0.dat

SignalSource.freq1=1227600000
SignalSource.gain1=50
SignalSource.samples1=0
SignalSource.dump1=false
SignalSource.dump_filename1=../data/signal_source1.dat

Multiple sources

A receiver can have more than one Signal Source delivering signal streams at the same time.

Examples of such configuration could be:

  • Two files, one for each band (such as in the case of NSL’s Stereo front-end);

  • Different antennas, working at the same band but with different RF front-ends;

  • Different front-ends sharing the same antenna.

Multiple sources
Simplified block diagram of a multi-source receiver of GPS L1 C/A and GPS L2C (M) signals.
Receiver.sources_count=2

Then:

Receiver.sources_count=2
...
SignalSource0.implementation=...
SignalSource1.implementation=...
...
SignalConditioner0.implementation=...
DataTypeAdaper0.implementation=...
InputFilter0.implementation=...
...
SignalConditioner1.implementation=...
DataTypeAdaper1.implementation=...
InputFilter1.implementation=...
...
Channels_1C.count=2
Channels_1B.count=2

...
; # CHANNEL CONNECTION
Channel0.SignalSource_ID=0
Channel1.SignalSource_ID=0
Channel2.SignalSource_ID=1
Channel3.SignalSource_ID=1

Channel0.signal=1C
Channel1.signal=1C
Channel2.signal=1B
Channel3.signal=1B
...

Updated:

Leave a Comment